Propeller assemblies typically include multiple blades mounted to a hub, which is rotated by the engine. The hub typically defines a housing that retains a blade root of a propeller blade along with a variety of other mechanisms, including a pitch control unit.
When a pitch control unit (PCU) is used, the blade root is rotatably retained within the housing for rotation about a span-wise axis of the blade and the PCU rotates the blade about the span-wise axis to optimize efficiency of thrust delivery. In this manner, the propeller can be designed to vary pitch in flight, to give optimum thrust, from takeoff and climb to cruise. Varying the pitch angle can allow the aircraft to maintain an optimal angle of attack, or maximum lift to drag ratio, on the propeller blades as aircraft speed varies.